The present invention relates to a semiconductor device having a radiation plate and a method of bonding a radiation plate.
A semiconductor device with a metal radiation plate bonded to a die pad to which a semiconductor element (IC chip) is die-bonded has been hitherto known as one type of semiconductor device having a semiconductor element (IC chip) having a relatively large heating value, such as a motor driver IC, a voice amplifying IC or the like. The die-bonding of the metal radiation plate to the die pad may be performed with adhesive agent, however, an ultrasonic bonding method is preferable because the adhesive agent has higher resistance to heat and thus it produces a thermal restriction in a fabrication process of semiconductor devices.
Here, the ultrasonic bonding of a die pad and a radiation plate with ultrasonic wave will be described with reference to FIG. 14.
First, a die pad 100 which is formed integrally with a lead frame (not shown) is put face down and mounted on a metal holding member 200 serving as a clamp member, and then a radiation plate 300 is superposed on the back surface of the die pad 100 as shown in FIG. 14. Thereafter, an ultrasonic horn 400 is disposed by a metal press member 500 serving as a clamp member on the radiation plate 300.
Subsequently, the radiation plate 300 is pressed from the upper side by the press member 500, and ultrasonic waves are applied through the press member 500 so that the ultrasonic wave acts on an interface between the die pad 100 and the radiation plate 300 to bond the die pad 100 and the radiation plate 300.
Each of the holding member 200 and the press member 500 is provided with a knurled face 600 to prevent the die pad 100 and the radiation plate 300 from slipping along each other, whereby the die pad 100a and the radiation plate 300 can be surely held.
After the above bonding process, a semiconductor element (IC chip) is die-bonded onto the die pad 100, and then the semiconductor element is wire-bonded to lead terminals of the lead frame. Thereafter, the result is molded in epoxy resin, and an undesired frame portion is removed to complete a semiconductor device.
In the above-described ultrasonic bonding method of the radiation plate 300, the knurled face 600 is provided on the upper surface of the holding member 200 for holding the die pad 100, and thus a plating layer formed on the surface of the die pad 100 is liable to be deeply scratched.
The deep scratches in the plating layer on the surface of the die pad 100 disturbs expansion (diffusion) of solder when the semiconductor element is die-bonded to the die pad 100, so that the semiconductor element cannot be surely bonded to the die pad.
In order to avoid the above problem, a method of omitting the knurled face 600 and holding the frame of the lead frame with pins or clamping the lead frame from upper and lower sides has been utilized. However, the vibration of the ultrasonic wave is transmitted to the pins or the clamping position, resulting in damage to the leads, or induces a slipping motion between the die pad 100 and the hold member 200 making the bonding unstable, or making the bonding strength lower even when the bonding succeeds.